The mediated electrosynthesis of H2O2 in acidic solutions (pH 0.9-3.0) was investigated in a three-phase, aqueous/organic/gaseous system using 2-ethyl-9,10-anthraquinone (EtAQ) as mediator (redox catalyst). The main hydrogen peroxide producing route is the in situ mediating cycle: EtAQ electroreduction-homogeneous oxidation of anthrahydroquinone (EtAQH(2)). The organic phase was composed of tributylphosphate solvent (TBP) with 0.2 M tetrabutylammonium perchlorate (TBAP) supporting electrolyte, 0.06 M tricaprylmethylammonium chloride (A336) surface active agent, and 0.1-0.2 M EtAQ mediator. Part I of this two part work deals with the physicochemical characteristics of the emulsion electrolyte (e.g., ionic conductivity, emulsion type, H2O2 partition between the aqueous and organic phases), and kinetic aspects (both electrode and homogenous) of the mediation cycle. Furthermore, batch electrosynthesis experiments are presented employing reticulated vitreous carbon cathodes (specific surface area 1800 m(2) m(-3)) operated at superficial current densities of 500-800 A m(-2). During 10 h batch electrolysis involving the emulsion mediated system with O-2 purge at 0.1 MPa pressure, H2O2 concentrations in the range 0.53-0.61 M were obtained in 0.1 M H2SO4 (pH 0.9) and 2 M Na2SO4 (acidified to pH 3). The corresponding apparent current efficiencies were from 46 to 68%. Part II of the present work describes investigations using flow-by fixed-bed electrochemical cells with co-current upward three-phase flow.